I designed this pop hole opener to lift a lever which allows a hinged door to open. It will lift a 10M tape with ease, but I would think that by using a stronger motor with its own battery and metal gears instead of plastic more weight would be possible. As I said befor all the parts can be bought fron Maplins.
Take one lead from the clock movement to one terminal on the motor, the other lead wraps around the base of the hour hand allowing the hand to rotate but still maintaining contact. Connect the other terminal on the motor to the switch. Then make a small hole just beneath the 6 on the dial, Take a small length of wire, connect one end to the switch and push the other end up though the hole under the 6. Sand the paint fron the tip of the hour hand to make good contact with the protruding wire beneath the 6. Because the hour hand moves very slowly it will remain in contact with the wire for much longer than necessary that is where the switch comes in. The string from the shaft goes down through a small hole in the arm of the switch then out of the box through a larger hole. Measure the distance you want your door or lever to move and tie a large knot in the string the same distance beneath the hole in the arm of the swich.
The way that the opener works is when the hour hand comes in contact with the wire the motor turns the string (or thin chain) around the shaft and when the desired height is reached the knot in the string pushes up against the small hole in the arm of the switch thus breaking contact.
The other end of the shaft protrudes through a hole in the box and when pushed in releases the worm from the gear allowing the string to be pulled down ready to be reset for the next day. Centralise the worm with the gear by using 1 or 2 small washers left of the gear and on the other side a spring is used to keep the gear in place and also allows the gear to spring back into place after being reset. The shaft, hole in the arm and the hole in the box should all be in line to prevent snagging of the string.
I am sure you can modify these instructions to make one that suits your own individual hen house. I will post a better picture, got camera but no lead.